The present invention relates to a non-aqueous electrolyte secondary battery using a lithium ion-conductive non-aqueous electrolyte in which a material capable of incorporating and releasing lithium ion is used as a positive electrode active material and a negative electrode active material, and a method of producing the same. More particularly, the invention relates to a novel secondary battery having high energy density and excellent charge and discharge characteristics at a voltage of about 1.5 V, a long cycle life, and a high reliability.
In recent years, following the remarkable development of portable type electronics equipment and devices, communication equipment and devices, and the like, various kinds of equipment and devices which require large current outputs for batteries as a power supply have been realized. It is therefore strongly desired to produce high energy density secondary batteries from the standpoint of economics, and compact size and light-weight of the devices. Further, with the necessity of devices having low voltage, the research and development for non-aqueous electrolyte secondary batteries having high energy density of about 1.5 V are conducted, a part of which is now in practical use.
Conventionally, titanium oxide and lithium-containing titanium oxide which are an active material constituting a negative electrode are described in Japanese Laid-Open patent application No. Sho 57-152669(hereinafter referred to as JP-A) and Japanese Patent Publication No. Sho 63-8588(hereinafter referred to as JP-B). Further, JP-B-63-1708 describes an example of a combination of a negative electrode using titanium oxide and a positive electrode using manganese dioxide. However, from the problems such that a voltage is 1 V, which is too low, and charge and discharge cycle life is short, it has not been in practical use. Furthermore, JP-A-6-275263 and JP-A-7-320784 describe a battery using improved lithium-containing titanium oxide as a negative electrode, but a positive electrode has not yet been investigated. That is, there has not been developed a secondary battery of about 1.5 V comprising a combination of a lithium-containing titanium oxide as a positive electrode and lithium-containing silicon oxide as a negative electrode.
Further, sulfides of iron have been investigated as a thermobattery or a primary battery, but a lithium ion type secondary battery using a non-aqueous solvent has not been investigated so much.
Nickel-cadmium batteries and nickel-hydrogen batteries which are conventional batteries of about 1.5 V have a small electric capacity. Therefore, there has been the problem that it is difficult to decrease its size. Further, 1.5 V batteries using lithium-containing titanium oxide as a negative electrode use manganese oxide or lithium-containing manganese oxide as a positive electrode. Therefore, the battery capacity is substantially determined by the capacity of manganese oxide or lithium-containing manganese oxide, and as a result, it has been difficult to further increase the capacity of the battery. In addition, it has been impossible to achieve decreasing inner resistance of the battery and increasing charge and discharge characteristics without sacrificing the capacity of the battery.
Lithium-containing titanium oxide and lithium-containing iron sulfide are a type that only lithium ions (cation) are input into and output from between layers of crystal, lattice positions or gaps among lattices of crystal by means of intercalation and deintercalation reactions and the like as is the case of metal chalcogenide such as TiS.sub.2, MoS.sub.2, NbSe.sub.3, and the like, and metal oxide such as MnO.sub.2, MoO.sub.3, V.sub.2 O.sub.5, Li.sub.x CoO.sub.2, Li.sub.x, NiO.sub.2, Li.sub.x Co.sub.1-y Ni.sub.y O.sub.2, Li.sub.x Mn.sub.2 O.sub.4, Li.sub.x MnO.sub.2, and the like. As a result of various improvements, lithium-containing titanium oxide has become to have a flat charge and discharge curve at about 1.5 V. However, its voltage is low as 1.5 V to lithium. Therefore, investigations have been made as a negative electrode, but investigations have not been made as a positive electrode.
Iron sulfide has investigated to apply to a thermal battery as a secondary battery. However, as a non-aqueous battery its capacity is large, but reversibility is poor, so that it has been investigated only as a primary battery. Thus, no great investigations have been made to lithium ion type secondary batteries using a non-aqueous solvent.
On the other hand, as a negative electrode active material when metallic lithium is used alone, its electrode voltage is poorest. Therefore, it is possible to produce about 1.5 V secondary battery if such is combined with a positive electrode using the above-described positive electrode active materials. However, there have been problems that dendrite or passive compounds are formed on lithium negative electrode due to charge and discharge, deterioration due to charge and discharge is large, and cycle life is short. In order to solve those problems, it has been proposed to use (1) alloys of lithium and other metals such as Al, Zn, Sn, Pb, Bi, Cd, or the like, (2) intercalation compounds or insertion compounds in which lithium ions are incorporated in a crystalline structure or an amorphous structure of, for example, carbonaceous materials obtained by clacining inorganic compounds such as WO.sub.2, MoO.sub.2, Fe.sub.2 O.sub.3, TiS.sub.2, Li.sub.x Co.sub.1-y Ni.sub.y O.sub.2, Li.sub.x WO.sub.y, or the like; graphite, or organic materials, and (3) materials capable of absorbing and releasing lithium ions, such as conductive polymers, e.g., polyacetone or polyacetylene and the like, having lithium ion doped therein.
However, when a battery is constituted by materials capable of incorporating and releasing lithium ions, other than the above-described metallic lithium, as a negative electrode active material, the electrode potential of those negative electrode active materials is nobler than the electrode potential of metallic lithium. Therefore, there is a disadvantage that the operating voltage is fairly lower as compared with the battery using metallic lithium alone as the negative electrode active material. For example, the operating voltage is lowered by 0.2 to 0.8 V when using alloys of lithium and Al, Zn, Pb, Sn, Bi, Cd, or the like, 0 to 1 V in a carbon-lithium intercalation compound, and 0.5 to 1.5 V when using a lithium ion insertion compound such as MoO.sub.2, WO.sub.2 or the like. For this reason, it was extremely difficult to form a 1.5 V secondary battery even by combining the above negative electrode with a positive electrode of lithium-containing titanium oxide or lithium-containing iron sulfide.
In addition, since elements other than lithium are involved as elements constituting the negative electrode, the capacity and energy density per volume and weight are considerably lowered. Further, where the above alloys (1) of lithium and other metals are used, the problem occurs that the utilization efficiency of lithium is low during charge and discharge, and repeating charge and discharge causes cracks or breaks in the electrode, resulting in a shorter cycle life. Where the battery uses the above lithium intercalation compound or insertion compound (2), deteriorations such as decay of the crystal structure and generation of irreversible substances arise due to excess charge and discharge, and further there is a disadvantage that since many compounds have higher (nobler) electrode-potential, the battery using the same has a lower output voltage. Where the battery uses the conductive polymers (3), there is the problem that the charge and discharge capacity, in particular, the charge and discharge capacity per unit volume, is small.
For those reasons, to obtain a secondary battery having high voltage, high energy density, excellent charge and discharge characteristics, and long cycle life, there is required a negative electrode active material having a larger effective charge and discharge capacity, that is, a larger amount of reversible absorption and release of lithium ions with a lower (baser) electrode potential for lithium but without deteriorations such as decay of the crystal structure and generation of irreversible substances and the like due to the absorption and release of the lithium ions during charging and discharging.
The present inventors previously found that lithium-containing silicon oxide represented by the compositional formula Li.sub.x SiO.sub.y (provided 0.ltoreq.x and 0&lt;y&lt;2) can incorporate and release lithium ions electrochemically, stably and repeatedly for a lithium standard electrode (metallic lithium) in the electrode potential range of 0 to 3 V in a non-aqueous electrolyte, has fairly high charge and discharge capacity in particularly a baser potential region of 0 to 1 V due to such a charge and discharge reaction, and can be used as an excellent negative electrode active material, and they filed applications (Japanese Patent Application Nos. Hei 4-265179, 5-35851, 5-162958, and the like).
It has been found in the present invention that a secondary battery of about 1.5 V having a capacity larger than that of the conventional capacity can be realized by using lithium-containing silicon oxide represented by the compositional formula Li.sub.x SiO.sub.y, wherein 0.ltoreq.x and0&lt;y&lt;2, as a negative electrode and also using lithium-containing titanium oxide or lithium-containing iron sulfide as a positive electrode. Since the lithium-containing silicon oxide has a greatly larger capacity for manganese oxide, a volume of the battery is not so increased even if conductive assistants are added in a relatively large amount. Therefore, inner resistance of the battery can be decreased, and the charge and discharge characteristics are improved.